If you delete all the email addresses on a domain using custom MX records, the domain's mail service will revert to DreamHost-hosted mail service and the custom MX records will be erased from the panel.

Extra step needed for Anti-Spam users

If you are switching to an MSP and currently using DreamHost's Anti-spam settings, the MX records for your domain will currently point to the anti-spam servers, not the actual servers where your mail accounts are and where the MSP needs to send mail.

Never give the MSP an MX record such as vade-in1.mail.dreamhost.com as doing so will cause your emails to disappear.

You need to figure out the correct MX records to give the MSP.

You can find your specific mail cluster in your panel on the (Panel > 'Support' > 'Data Centers') page. View the following article for instructions on how to locate your mail servername.